The design and fabrication of a high-Tc DC squid gradiometer

Knox-Davies, Evan Carl (1999-12)

Thesis (M.Ing) -- University of Stellenbosch, 1999.

Thesis

ENGLISH SUMMARY: Superconducting quantum interference devices (SQUIDs) are very sensitive magnetic field sensors, finding applications in fundamental physics, non-destructive testing, biomagnetism and geophysics. This thesis describes how to design and fabricate a high-temperature dc SQUID gradiometer for operation at 77 K. A design procedure for directly coupled SQUID gradiometers which incorporates recommendations and typical parameter values from literature is described. It is used to design a gradiometer and estimate its electrical parameters. The effect of thermal noise on the SQUID sensitivity as well as the current opinion on the mechanism and contribution of flicker noise are discussed. A number of commonly used SQUID read-out schemes and their respective merits are discussed. The SQUID fabrication process and the optimisations necessary to perfect the process are described in some detail. A thin film of YBa2Cu307 , a high-temperature superconductor, is prepared by pulsed laser deposition on an MgO bicrystal substrate. The film is then patterned by conventional photolithography and wet etching to produce the desired circuit shape and form Josephson junctions on the bicrystal's grain boundary. Gold contact pads are deposited through a mask by pulsed laser deposition. A simpler version of the same fabrication process can be used to manufacture microwave filters. Two attempts were made at constructing a Josephson junction test circuit but at no point did measurements of the fabricated junctions ever detect Josephson behaviour. This can be directly attributed to an inferior quality thin film. It was not attempted to fabricate the SQUID gradiometer because the successful operation of Josephson junctions could not be demonstrated. Although the testing of the junctions did not produce positive results, the entire fabrication process, except for the deposition of a good quality film, has been demonstrated to be a success. Once a good quality superconducting film can be deposited, it should be straightforward to fabricate working Josephson junctions and SQUIDs.

AFRIKAANSE OPSOMMING: 'n SQUID (supergeleidende kwantum interferensie toestel) is 'n baie sensltlewe magnetiese-veld sensor wat verskeie toepassings het, soos byvoorbeeld in fundamentele fisika, nie-destruktiewe metings, bio-magnetisme en geofisika. Hierdie tesis beskryf hoe om 'n hoe-temperatuur gs SQUID magnetiese gradientmeter vir operasie by 77 K te ontwerp. 'n Ontwerpsprosedure vir direk-gekoppelde SQUID gradientmeters word daargestel. Tipiese parameter waardes en voorstelle wat in die literatuur beskryf word, word benut in hierdie prosedure, wat die ontwerp van 'n gradientmeter en die beraming van die elektriese eienskappe behels. Termiese ruis word in ag gene em deur die effek daarvan op die sensitiwiteit van die SQUID te beskryf. Die mees onlangse idees vir die verskynsel en effek van flikker ruis word ook bespreek. Algemene SQUID uitleesskemas en die meriete van elk word ook verduidelik. Baie aandag word in die tesis bestee aan die besonderhede van die vervaardiging van 'n SQUID, sowel as die optimeringsstappe wat gevolg is. 'n YBa2Cu307 dunfilm ('n hoe-temperatuur supergeleier), word deur middel van gepulsde laserdeposisie op 'n MgO bikristal substraat gedeponeer. Die film word dan deur standaard fotolitografie en nat etsing gevorm om Josephsonvlakke op die bikristal se greinvlak sowel as die gewensde. stroombaanuitleg te vorm. Goud kontakte word deur middel van geplflsde laserdeposisie deur 'n masker gedeponeer. 'n Eenvoudiger weergawe van bogenoemde vervaardigingsproses kan ook gebruik word om mikrogolffilters te produseer. Twee pogings is aangewend om 'n Josephson vlak toetsbaan te bou, maar geen Josephson gedrag kon deur metings waargeneem word nie. Dit kan definitief toegeskryf word aan die swak kwaliteit van die dunfilm. 'n SQUID gradientmeter is nie vervaardig nie, aangesien die korrekte werking van die Josephson vlakke nie waargeneem kon word nie. Buiten vir die feit dat die toetsing van die Josephson vlakke nie positiewe resultate gelewer het nie, was die fabrikasieproses, met uitsondering van 'n goeie film deposisie, duidelik In sukses. Sodra hierdie probleem oorbrug kan word, behoort dit eenvoudig te wees om werkende Josephson vlakke en SQUIDs te vervaardig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/51454
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